Many experiments have demonstrated that early experience dramatically influences brain and behavior development. However, little is known regarding the chain of cellular events underlying this process. Thus, identification of cellular mechanisms whereby an organism's interaction with its environment influences brain development is among the most challenging and the most important problems in modern neurobiology. The proposed research is part of a systematic research program which endeavors to understand the influence of neuronal activation on the ontogeny of neural structure and function. The brain stem auditory pathways of the avian embryo and hatchling have been chosen as the "model system" to use because several characteristics of their input-output relationships make them uniquely well suited to such investigations. In experiments already completed or underway, direct or indirect deafferentation has been used to assay the influence of innervation on neuronal morphology. In the proposed experiments, receptor cell destruction, specific environmental sound deprivation and limited acoustic experience will be used to more fully understand the influence of synaptic use on the ontogeny of neuronal morphology and auditory information processing. These investigations are fundamental to eventually understanding the cellular chain of events underlying experiential influences on neural and behavioral development. The additional clinical importance of these studies is that they will further identify the central nervous system changes resulting from congential auditory deficits and the possibilities for restoration of normal function.